Inhibitory mechanisms are known to be crucial in motor control, epilepsy, anaesthesia, and many other normal and abnormal aspects of a neural function. At present, however, analyses of inhibitory mechanisms are complicated by uncertainties concerning the anatomy of inhibitory circuits, the location of inhibitory synapses on postsynaptic neurons, and the candidate neurotransmitters used by different kinds of inhibitory interneurons. In the segmental motor system two of the major inhibitory inputs to motoneurons are mediated by identifiable interneurons - Renshaw cells (mediating recurrent inhibition) and Ia inhibitory interneurons (mediating reciprocal inhibition). The continuing long term goal of this proposal is to better understand the physiological role of Renshaw cells. To achieve this, it is necessary to compare their properties with the mechanisms of action of at least one other inhibitory input to motoneurons. The Ia inhibitory interneurons are good candidates for this purpose as their own activity is modulated by Renshaw cell actions, thus directly linking the two systems in controlling motor output. The experiments will use techniques which combine intracellular recording and staining of identified inhibitory interneurons and their postsynaptic targets, and subsequent analysis, using light and electron microscopy and postembedding immunogold techniques. The specific aims are to continue the detailed analysis of Renshaw cell connections to motorneurons, with particular emphasis on how topography and motoneuron type might influence the incidence and distribution of the inhibitory synapses. A similar type of study will be performed on the Ia inhibitory system. For both inhibitory systems, detailed ultrastructural and immunocytochemical analyses will be performed to determine their synaptic relationships and putative neurotransmitters.